1. Field of the Invention
This invention is for a cable filled with a gel chemical composition that, when exposed to changes in ambient temperature, substantially maintains its nominal mutual capacitance. The composition is a water absorbent thixotropic gel (ATG) that is activated by moisture to absorb water and yet can be used to protect components from water damage.
The composition is incorporated into the cable, either between conductors in a bundle and/or between the bundles of conductors contained in, for instance, a telecommunications cable. Regardless of the use of the cable, not only does the composition prevent the entry of water, but the composition also eliminates electrical shorts caused by water contact with the conductors in cables, such as telephone cables, which carry a small direct current, thereby attenuating the short and restoring full current flow through the conductors.
2. Description of the Prior Art
Communications cables such as telephone lines are made up of a multitude of pairs of conducting wires, typically copper wire, which are insulated from each other with a thin layer of a thermoplastic resin and bundled by an insulating material. Bundles of insulated pairs of conducting wires are then wrapped with a sheath of plastic, paper wrapping or other material, into a cable. A filler such as a petroleum gel is added to many cables inside the cable cover to fill the interstitial spaces and retard water migration therein.
In recognition of the essential nature of the ability of telecommunications cable to be able to withstand exposure to water, the industry has promulgated certain performance standards which the cable must achieve. In particular, one standard requires that a three foot long section of cable not pass water when maintained under a column of water three feet high (e.g., a "three foot water head"). The industry is currently considering a change in that standard to require that a section of cable eight feet long not pass water when subjected to twelve feet of water head for twenty four hours.
As noted above, the prior art discloses protection of telecommunications cable against water invasion by filling the spaces between the wrapped bundles of conducting wires inside the cable (referred to as the filling zone) with compounds such as polyethylene-petroleum jelly (PEPJ) and oil-extended thermoplastic rubbers (ETPR), the latter being widely used by AT&T and the Bell regional operating companies in the United States and sold under the trademark FLEX-GEL. Certain patents also describe telecommunications cables including water swellable polymers such as polyvinyl alcohol, polyacrylamides, or cellulose derivatives, which are applied to bundle wrappings or contained in "moisture barriers" which are spaced internally along the length of the cable. Such cables are, however, characterized by a number of limitations and disadvantages. In the case of those which include a polymer which swells in the presence of water, the polymer is typically provided in a granular or powder form. As such, distribution of the polymer throughout the cable is problematical. If not distributed evenly throughout the cable, effective water absorbance is not assured. Further, when insufficient quantities of polymer are present, the ability of the swollen polymer to block water migration becomes problematical. Another problem is that many water-absorbent polymers, especially in the case of cellulose derivatives and other naturally-occurring polymers, are susceptible to bacterial attack, resulting in production of acids and other by-products which can damage or degrade the components of the cable.
Perhaps more importantly, on contact with water, powders alter the electrical characteristics of the cable. Using smaller quantities of the powder in the cable so as to decrease that effect compromises the water blockage capabilities of the powder. Further, certain swelling agents such as polyvinyl alcohols and polyacrylamides do not swell quickly enough in cold water to effect proper water blockage when the bundle is only partially filled, while filling the bundle completely with such agents is prohibitively expensive and causes problems with swelling in the confined space when contacted by water.
ETPR filling compounds are also characterized by a number of disadvantages and/or limitations. For instance, ETPRs must be heated to achieve a liquid state for handling and filling of the cable, increasing cost and creating logistical problems during storage and transport of the material. Cable is filled at about 230.degree. F. and under pressure such that the filling operation is relatively dangerous and thermal contraction after filling results in the formation of voids which can serve as paths for water migration. Further, only recently have ETPRs been available which can be used in aerial cables; the temperatures in the cable resulting from ambient temperature and exposure to sunlight caused previous ETPRs to liquify and drip out of the cable. Recently issued U.S. Pat. No. 4,870,117 is directed to a filling compound which is stated to maintain its gel state at temperatures up to 80.degree. C. (e.g., the temperature to which aerial cables may be subjected), but reports from the field indicate that such cables may not be performing as expected. Further, so far as is known, cable including this material cannot meet the proposed 12 feet/24 hour industry standard for resistance to water penetration.
Another problem with the use of ETPRs which has recently come to light is the cracking of the foam-skin polyethylene used to insulate the wires of the ETPR-filled cable. See, for instance, T. N. Bowmer, "Cracking of Foam-Skin Polyethylene Insulation in Pedestals, " Proceed. 37th Int. Wire & Cable Symp. 475 (1988). The response of the industry to this problem was to increase anti-oxidation stabilizer content to obtain improved foam-skin life expectancy. That approach, however, does not address the fundamental issue of the compatibility (or incompatibility) of the resin, stabilizer(s) and/or filling compounds.
Petroleum gels are generally used as filling compounds, in part because all known substitutes suffer from one or more disadvantages which limit their utility such that petroleum gels represent the least expensive alternative. However, petroleum gels are generally characterized by many of the same disadvantages of ETPRs.
In short, in spite of a continuing and long-felt need, and in spite of the many attempts which have been made to solve these problems, there is still a need for a water resistant cable, and further, for a filled cable having stable capacitance relative to temperature change and which is less susceptible to the effects of long term aging. The ideal cable would maintain constant electrical parameters regardless of the ambient conditions such as temperature or the presence or absence of moisture and as the cable ages.